We studied the relation between the antipapain activity of cysteine proteinase inhibitors (CPI) and immunohistochemical staining for cystatin C, using anti-chicken cystatin antibodies, in the colorectal cancer tissues. In primary tumour tissues immuno-peroxidase reactivity was present in the cytoplasm and on the cell surface membranes. Sections of non malignant tissues showed no staining. The percentages of positive staining were greater for adenocarcinoma than carcinoma,100% and 77% respectively. Antipapain activity which was increased in malignant tissues in comparison to control, rose successively from well differentiated carcinomas through moderately to poor differentiated. Invasive adenocarcinomas had higher antipapain activity than noninvasive ones. The results indicated that immunohistochemical detection of cystatin using anti-chicken cystatin antibodies could be useful in studying the prognostic significance of cystatin C expression in colorectal cancer.